【作者】 徐秀华；

【导师】 肖汉宁；

【作者基本信息】 湖南大学 ， 材料学， 2011， 硕士

【摘要】 六硼化镧（LaB₆）材料具有电导率高、稳定性好、高温下蒸发率低、发射电流密度大等优异性能,已在电子显微镜、电子束焊接、放电管等需要大发射电流的领域获得应用。因其所具有的优异特性以及广泛的应用领域,大量科研机构都陆续开展了LaB₆材料的研究。然而,为了制备高性能的LaB₆多晶材料,首先需要制得纯度高、粒度分布均匀的LaB₆粉末。因此,本文旨在研究纯度高、烧结性能良好、粒度分布均匀的LaB₆粉末的低成本制备技术,为制备高性能LaB₆多晶体及其复合材料奠定基础。以La₂O₃（D50<0.2μm）和B4C（D50=3.70μm）为原料,在常压下合成了LaB₆粉末。计算了常压合成LaB₆的热力学条件,采用XRD、SEM、激光粒度分析表征了不同温度、保温时间和不同粒径B4C原料合成的LaB₆粉末的物相组成、颗粒形貌和粒度分布,探讨了LaB₆粉末的合成反应机理。结果表明,常压下1650℃保温2h的产物经酸洗后,能得到纯度为99.22%、平均粒径为15.86μm、粒度分布均匀、具有立方体结构的LaB₆粉末。当La₂O₃颗粒尺寸远小于B4C时,LaB₆首先在B4C表面生成,随温度升高和保温时间延长,未反应的La₂O₃和LaBO3通过LaB₆壳不断扩散到B4C核表面直至反应完全。温度和保温时间对合成LaB₆颗粒的影响没有B4C原料明显,合成LaB₆粉末的初始形貌和尺寸主要取决于反应物B4C原料。本文采用热压烧结技术制备了LaB₆多晶体,系统研究了烧结温度、保温时间、外加压力和粉体粒度对LaB₆多晶体性能的影响,初步探讨了LaB₆多晶体的热压烧结机理。研究结果表明:以自制的纯度达99.22%、平均粒径为15.86μm的LaB₆粉末为原料,在2050℃、30MPa条件下保温1h,制备的LaB₆多晶体的相对致密度达到96.32%,抗弯强度和显微硬度分别达到153.2MPa和16.27GPa。烧结温度、保温时间和外加压力的提高,有利于LaB₆多晶体的烧结致密化,但过高的烧结和延长保温时间均不利于材料的进一步致密化,反而会使晶粒异常长大,材料性能急剧下降。采用较小粒径的LaB₆粉末有利于增加致密化烧结速率,也有利于减小气孔尺寸和气孔率,能有效提高样品的烧结密度。更多还原

【Abstract】 Lanthanum hexaboride has been receiving more and more attentions as a high performance material because of its unique properties, such as high electric conductivity, good chemical stability, low rate of evaporation at high temperature and high emission current density. LaB₆ is widely used as cathode parts in demanding huge emission current field instrument, such us electron microscope, electron beam welding industry, discharge tube industry and so on. Due to its special properties and broad application prospect, the type of materials research have been carried out by many institutions. However, in order to preparation high performance LaB₆ polycrystalline, high purity and uniform particle size distribution of LaB₆ powder is been needed at first. So the low cost preparation technology of high purity and good sintering performance of LaB₆ powder were investigated in this paper, which may establish the academic base for the preparation of high performance LaB₆ polycrystalline and composites.The lanthanum hexaboride powders （LaB₆） were synthesized under atmospheric pressure by using La₂O₃ （D50<0.2μm） and B4C （D50=3.70μm） powder as the precursors. The reaction thermodynamic data for preparing LaB₆ was calculated. The effects of temperature, holding time and different particlesize of B4C on the phase composition, morphology, particle size and distribution of LaB₆ powders were characterized by X-ray diffraction（XRD）, scanning electron microscope（SEM） and Laser particle size anslyzer. The reaction mechanism was also investigated. Results showed that LaB₆ powders can be obtained under atmospheric pressure at 1650℃for 2h with 99.22% purity, D50=15.86μm, uniform particle size distribution and cube structure. When the particle size of La₂O₃ powder is much smaller than that of B4C particles, the growth of LaB₆ starts on the surface of B4C particles, then the residual La₂O₃ and LaBO3 will diffuse through LaB₆ shell onto B4C core until the reaction is completely. The influence of temperature and holding time on the synthesis of LaB₆ powder were not obviously than B4C raw materials, The initial shape and particle size of LaB₆ powders were depended on that of B4C particles.The LaB₆ polycrystalline was produced by using hot-pressing method. The influence of temperature, holding time, hot-pressure and powder particle-size on the LaB₆ polycrystalline performance were system characterized. The LaB₆ polycrystalline hot-pressing sintering mechanism was also investigated. Results show that high performance LaB₆ polycrystalline obtained under 30MPa hot-pressure, at 2050℃for 1h by using self-made LaB₆ powder with 99.22% purity and D50=15.86μm, which relative density reached 96.32%, bending strength and microhardness achieved respectively 153.2MPa and 16.27GPa. With the increasing of temperature, holding time and hot pressure are in favor of LaB₆ polycrystalline sintering densification. However, exorbitant sintering temperature and prolong holding time would lead to grain abnormal grow and material properties sharply decline, which is disadvantage of materials further densification. It is benefit of increasing sintering densification rate, decreasing the stomata size and porosity by using smaller particle-size LaB₆ powders as sintering materials.更多还原